With regard to zirconium phosphate, there are amorphous form and crystalline forms having a 2-dimensional lamellar structure or a 3-dimensional network structure. Among them, a hexagonal zirconium phosphate having a 3-dimensional network structure is excellent in terms of heat resistance, chemical resistance, radiation resistance, low thermal expansion, etc., and the application thereof to immobilization of radioactive waste, solid electrolytes, gas absorption/separation agents, catalysts, antimicrobial agent starting materials, low thermal expansion fillers, etc. has been investigated.
Hexagonal zirconium phosphates with various components are known so far, and examples thereof include NaZr2(PO4)3, CaZr4(PO4)6, and KZr2(PO4)3 (ref. e.g. Patent Document 1, Non-Patent Document 1).
In general, with regard to a method for synthesizing these zirconium phosphates, a calcination method in which synthesis is carried out by mixing starting materials by a dry method and then calcining using a calcining furnace, etc. at 1000° C. or greater (ref. e.g. Patent Document 2, Patent Document 4), a hydrothermal method in which synthesis is carried out by mixing starting materials in water or in a water-containing state and then heating while applying pressure, a wet method in which synthesis is carried out by mixing starting materials in water and then heating at normal pressure (ref. e.g. Patent Document 3), etc. are known.
Among these synthetic methods, in the calcination method, since uniform mixing of starting materials is not easy it is difficult to obtain a zirconium phosphate having a uniform composition. Furthermore, after crystallization by calcining, it is necessary to carry out classification after grinding in order to obtain a powder having a specific particle size, and there are therefore the problems in terms of quality and productivity that the shape and the particle size of the powder are not easily stabilized and the yield is low. In particular, since the hardness of a highly crystalline zirconium phosphate is high, in order to obtain a powder with high purity, there is the problem that grinding conditions such as selection of materials for a grinder and grinding time are restricted.
On the other hand, in accordance with the wet method and the hydrothermal method, although it is easy to obtain a uniform zirconium phosphate powder, that obtained is mainly a microparticulate crystalline powder having a size of 1 μm or less, and it is technically difficult to obtain mainly a crystalline powder having a particle size of greater than 1 μm. Furthermore, using sieving of large particles from microparticles obtained by the wet method or the hydrothermal method results in high cost and is not economical.    [Patent Document 1] Japanese Patent No. 2767276    [Patent Document 2] JP-A-2000-290007 (JP-A denotes a Japanese unexamined patent application publication)    [Patent Document 3] Japanese Patent No. 3211215    [Patent Document 4] JP-A-3-83905    [Non-Patent Document 1] Shigetaka Ohota and Iwao Yamai, ‘Teinetsubocho KZr2(PO4)3 Seramikku no Sakusei’ (Preparation of low thermal expansion KZr2(PO4)3 ceramic), Journal of the Ceramic Society of Japan, 1987, Vol. 95, No. 5, p 531-537.